Pfizer-BioNTech said Thursday that it is beginning clinical trials of its Covid-19 vaccine in pregnant women, the first such trials to include expectant mothers in the U.S. The drugmaker aims to enroll about 4,000 pregnant women in the trials, which will include participants in the U.S. as well as Argentina, Brazil, Canada, Chile, Mozambique, South Africa, Spain and the U.K. Women over 18 and who are 24 to 34 weeks into their pregnancy will be eligible. The first doses will be administered in the U.S., Pfizer said. Dr. Brenna Hughes, a maternal-fetal medicine specialist at Duke Health in Durham, North Carolina, said she "absolutely applauds" Pfizer's study of its vaccine in pregnant women. "Any data to help reassure pregnant patients that the vaccine is safe for them is desperately needed," Hughes, a member of the American College of Obstetricians and Gynecologists, said. "We are proud to start this study in pregnant women and continue to gather the evidence on safety and efficacy to potentially support the use of the vaccine by important subpopulations," Dr. William Gruber, senior vice president of Vaccine Clinical Research at Pfizer, said in a statement. Some of the women will get the real shots, while others get a placebo. They won't know which kind they received until after giving birth. At that point, women who got the placebo will be offered the vaccine. Researchers will monitor for any negative side effects in women, including miscarriage. There are some preliminary data on safety during pregnancy, because some women in earlier studies of the Covid-19 vaccine became pregnant while they participated in clinical trials. "From everything that we're seeing so far from pregnant women who've had the vaccine, there are no red flags," Stacey Stewart, president of the March of Dimes, said. The pregnant women who participated in the clinical trials of the Covid-19 vaccine are really admirable. After all, the impact of the vaccine on pregnant women is unknown. This is good news for us ordinary people.